Ceramic matrix composites (CMCs) combine a ceramic fiber (e.g., SiC) in a ceramic matrix (e.g., SiC). CMCs are useful for certain high-temperature applications, and may enable equipment to endure higher operating temperatures, which can be advantageous.
For example, hot section components have been manufactured from Ni-based super-alloys for commercial and military turbine engines. Through the past several decades, many advances in materials processing, composition, and design of metal components have led to steadily increased thermodynamic performance. To achieve further dramatic improvements, higher temperature non-metallic approaches need to be pursued and implemented. Experts in the field of turbine engines have concluded that ceramic matrix composites represent a breakthrough opportunity for improving turbine engine performance. It has been estimated that use of CMCs in the hot zone may simultaneously and substantially reduce NOx emissions, reduce specific fuel consumption (SFC), and increase engine thrust to weight ratios.
In pursuit of these and other benefits, turbine engine manufacturers have been investing in CMCs for turbine engine applications, both civilian and military. In addition to the benefits mentioned above, CMC components for turbine engines are believed to reduce engine weight, operate at temperatures as high as 2700° F., decrease containment requirements, and support inspection/replacement intervals as high as 6000 hours. CMCs are being considered for components such as low pressure turbine vanes and blades, high pressure turbine vanes, air seals, and combustors.
Conventional CMC manufacturing methods have some significant drawbacks. The two most widely studied processes are chemical vapor infiltration (CVI) and polymer-impregnation-pyrolysis (PIP). As has been noted by others, there are inherent microstructural differences associated with the two processes. FIG. 1 shows electron-microscope images 100 of a PIP-fabricated CMC from the prior art. The image at left 102 shows a cross-section of the PIP-fabricated CMC taken parallel to the fiber grain. The images at right 104, 106 show cross-sections of the PIP-fabricated CMC taken perpendicular to the grain direction, with the upper image 106 at a higher magnification. The images show fine dispersed micro cracks and both closed and open porosity. FIG. 2 shows electron-microscope images 200 of a CVI-fabricated CMC from the prior art. The image at left 202 shows a cross-section of the CVI-fabricated CMC taken parallel to the fiber grain. The images at right 204, 206 show cross-sections of the CVI-fabricated CMC taken perpendicular to the grain direction, with the upper image 206 at a higher magnification. These images show that the CVI fabrication creates larger pockets of closed porosity relative to PIP, and generally no open porosity. These characteristics of PIP and CVI manufacturing lead to CMCs with less than optimal characteristics for strength and toughness.
Melt infiltration (MI) manufacturing of SiC/SiC CMCs has been investigated for many years. The process, for both carbon fiber preforms and later SiC fiber preforms was pioneered by BF Goodrich (now owned by United Technologies). The application of MI-SiC for turbine engines has been the subject of intense research by NASA and others. Some key considerations include residual stress retained in complex parts resulting from cooling of MI-SiC CMC parts. Additionally, retention of free silicon and free carbon has been an issue with the MI process, in which molten silicon is infiltrated into the fiber preform after it has been infused with a carbon source. Due to the high temperature of the process itself and lengthy heat treatment times, only a limited number of ceramic fibers are stable, such as Hi-Nicalon Type S and Sylramic, both forms of high temperature silicon carbide ceramic fibers.
It would be desirable, therefore, to develop new methods and technology that overcome these and other limitations of the prior art, for fabricating ceramic matric composites.